This gist shows how to setup a custom error_page on nginx when using error_page as a way to redirect requests to a upstream server. You need to set recursive_error_pages on for error_page to work like expected.

nginx.conf

server {
	recursive_error_pages on;
 
	location / {
		error_page 404 = @memcached;
	}
	
	location @memcached {
		proxy_pass http://upstreamserver;
	}
	
	error_page 500 502 503 504 /50x.html;
	location = /50x.html {
	     root /var/www/error-pages/;
	}
}